The invention relates to a diffractive security element which is divided into surface portions with an optically effective structure of interfaces embedded between two layers of a composite of plastic material.
Diffractive security elements of that kind are used for the verification of articles such as banknotes, passes and identity cards of all kinds, valuable documents and so forth in order to be able to establish the authenticity of the article without involving a high level of cost. When the article is issued the diffractive security element is fixedly joined thereto, in the form of a stamp portion cut from a thin layer composite.
Diffractive security elements of the kind set forth in the opening part of this specification are known from EP 0 105 099 A1 and EP 0 375 833 A1. Those security elements include a pattern of surface elements which are arranged in a mosaic-like fashion and which have a diffraction grating. The diffraction gratings are azimuthally predetermined in such a way that, upon a rotary movement, the visible pattern produced by diffracted light performs a predetermined sequence of movements.
U.S. Pat. No. 4,856,857 describes the structure of transparent security elements with impressed microscopically fine relief structures. Those diffractive security elements generally comprise a portion of a thin layer composite of plastic material. The interface between two of the layers has microscopically fine reliefs of light-diffracting structures. To enhance reflectivity the interface between the two layers is covered with a mostly metallic reflection layer. The structure of the thin layer composite and the materials which can be used for that purpose are described for example in U.S. Pat. No. 4,856,857 and WO 99/47983. It is known from DE 33 08 831 A1 for the thin layer composite to be applied to an article by means of a carrier film.
The disadvantage of the known diffractive security elements lies in the difficulty of visually recognising complicated, optically varying patterns in a narrow solid angle and the extremely high level of surface brightness, at which a surface element occupied by a diffraction grating is visible to an observer. The high level of surface brightness can also make it difficult to recognise the shape of the surface element.
A security element which is simple to recognise is known from WO 83/00395. It comprises a diffractive subtractive color filter which, upon illumination with for example daylight, in a viewing direction, reflects red light and, after rotation of the security element in the plane thereof through 90°, reflects light of another color. The security element comprises fine bars, embedded in plastic material, the bars being of a transparent dielectric with a refractive index which is much greater than that of the plastic material. The bars form a grating structure with a spatial frequency of 2500 lines/mm and reflect in the zero diffraction order red light with a very high level of efficiency if the white light incident on the bar structure is polarised in such a way that the E-vector of the incident light is oriented in parallel relationship with the bars. For spatial frequencies of 3100 lines/mm the bar structure reflects green light in the zero diffraction order, while for even higher spatial frequencies the reflected color goes into the blue range in the spectrum. According to van Renesse, Optical Document Security, 2nd Edition, pages 274–277, ISBN 0-89006-982-4, such structures are difficult to produce inexpensively in large amounts.
U.S. Pat. No. 4,426,130 describes transparent, reflecting sinusoidal phase grating structures. The phase grating structures are so designed that they have the highest possible level of diffraction efficiency in one of the two first diffraction orders.